<template>
	<scroll-view class="scroll-y" scroll-y="true" @scrolltolower="tolower" :scroll-top="scrollTop">
		<view class="u-waterfall" id="list">
			<view id="u-left-column" class="u-column">
				<slot name="left" :leftList="leftList"></slot>
			</view>
			<view id="u-right-column" class="u-column">
				<slot name="right" :rightList="rightList"></slot>
			</view>
		</view>
	</scroll-view>
</template>

<script>
	export default {
		name: "waterfall",
		props: {
			value: {
				// 瀑布流数据
				type: Array,
				required: true,
				default: function() {
					return [];
				}
			},
			scrolltolower: {
				type: Function,
				default: () => {}
			}
		},
		data() {
			return {
				leftList: [],
				rightList: [],
				tempList: [],
				scrollTop: 0,
			}
		},
		watch: {
			copyFlowList(nVal, oVal) {
				this.tempList = this.cloneData(this.copyFlowList);
				this.splitData();
			}
		},
		mounted() {
			this.tempList = this.cloneData(this.copyFlowList);
			this.splitData();
			// this.$on('clearWaterFall', this.clear)
		},
		computed: {
			// 破坏flowList变量的引用，否则watch的结果新旧值是一样的
			copyFlowList() {
				return this.cloneData(this.value);
			}
		},
		methods: {
			async splitData() {
				if (!this.tempList.length) return;
				let leftRect = await this.$uGetRect('#u-left-column');
				let rightRect = await this.$uGetRect('#u-right-column');
				// 如果左边小于或等于右边，就添加到左边，否则添加到右边
				let item = this.tempList[0];
				// 解决多次快速上拉后，可能数据会乱的问题，因为经过上面的两个await节点查询阻塞一定时间，加上后面的定时器干扰
				// 数组可能变成[]，导致此item值可能为undefined
				if (!item) return;
				if (leftRect.height < rightRect.height) {
					this.leftList.push(item);
				} else if (leftRect.height > rightRect.height) {
					this.rightList.push(item);
				} else {
					// 这里是为了保证第一和第二张添加时，左右都能有内容
					// 因为添加第一张，实际队列的高度可能还是0，这时需要根据队列元素长度判断下一个该放哪边
					if (this.leftList.length <= this.rightList.length) {
						this.leftList.push(item);
					} else {
						this.rightList.push(item);
					}
				}
				// 移除临时列表的第一项
				this.tempList.splice(0, 1);
				// 如果临时数组还有数据，继续循环
				if (this.tempList.length) {
					this.splitData();
					return
				}
			},
			// 复制而不是引用对象和数组
			cloneData(data) {
				return JSON.parse(JSON.stringify(data));
			},
			tolower(e) {
				this.scrolltolower()
			},
			clear() {
				this.leftList = []
				this.rightList = []
			}
		}
	}
</script>

<style lang="scss" scoped>
	@mixin vue-flex($direction: row) {
		/* #ifndef APP-NVUE */
		display: flex;
		flex-direction: $direction;
		/* #endif */
	}

	.scroll-y {
		height: 78vh;
		margin-top: 18px;
	}

	.u-waterfall {
		@include vue-flex;
		flex-direction: row;
		align-items: flex-start;
	}

	.u-column {
		@include vue-flex;
		flex: 1;
		flex-direction: column;
		height: auto;
		width: 45vw;
		word-break: break-all;
	}
</style>
